Coordinate positioning machine

ABSTRACT

A coordinate positioning machine comprises a fixed structure including a table and a supporting frame, rigidly connected together. A movable arm is suspended from the frame by means of three powered telescopic struts, each of which is universally pivotally connected to both the arm and the frame. As a consequence, the movable arm is able to move with three rotational degrees of freedom. Movement of the arm with each of these degrees of freedom is constrained by a passive device, connected to the arm and the fixed structure, and which eliminates all rotational movement of the arm, while simultaneously permitting translation thereof.

This is a division of application Ser. No. 09/161,284, filed Sep. 28,1998, now U.S. Pat. No. 6,145,405 which in turn is a continuation ofapplication Ser. No. 08/685,097, filed Jul. 22, 1996, now U.S. Pat. No.5,813,287, which in turn is a Continuation-in-Part of application Ser.No. 08/396,721, filed Mar. 1, 1995, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to a coordinate positioning machine such as amachine tool, inspection robot, or coordinate measuring machine.Coordinate positioning machines include a table for supporting an objectupon which the machine is operating, and an arm movable relative to thetable, typically with three linear degrees of freedom, which carries anoperating module such as a cutting tool, an inspection probe, or awelding arm, for example.

2. Description of Related Art

Conventional coordinate positioning machines support the movable armeither, in the case of a robot, by a plurality of serially mountedrotatable joints, or, in the case of a machine tool and coordinatemeasuring machine, on a plurality of serially mounted linear guideways.In each case the serial mounting of the movable arm results in differentinertial loads on the machine when the movable arm is displaced indifferent directions, due to the differing number of moving machineparts which must be displaced to enable such movement. Additionally, anyforce applied to the movable arm, for example via the operating module,will result in bending moments being applied to at least part of thestructure which supports the arm.

In an alternative form of coordinate positioning machine, the movablearm is supported by a plurality of members, each of which is connectedto the mechanical earth of the machine, such as the table, for example.Machines of this type are known from e.g. International PatentApplication Nos. WO91/03145 (Kearney & Trecker) and WO92/17313 (GeodeticMachines), in European Patent Application No. 534585 (Ingersoll), andU.S. Pat. No. 4,732,525, and typically include a movable arm, supportedrelative to a fixed, or “earthed” structure by means of a plurality oftelescopic struts. Movement of the movable arm is achieved by extensionand, where appropriate, contraction of one or more of the struts. Afurther type of coordinate positioning machine is shown in U.S. Pat. No.4,976,582.

SUMMARY OF THE INVENTION

The present invention provides a coordinate positioning machine having:a fixed structure; an arm, supported for movement relative to the fixedstructure, upon which an operating module may be mounted; the arm beingsupported relative to the fixed structure by three telescopic struts,each having a motor which is actuable to increase or decrease the lengthof the corresponding strut; the struts being universally pivotallyconnected at one end to said arm, and at the other end to said fixedstructure, the arm thereby possessing three rotational degrees offreedom for any given combination of lengths of the three struts;constraining means acting between the fixed structure and the arm, forconstraining movement of the arm with each of said three rotationaldegrees of freedom to within predetermined limits, while simultaneouslypermitting translation of said arm with three linear degrees of freedom,and including at least one passive device which eliminates one of saidrotational degrees of freedom.

In one preferred embodiment, the constraining means is entirely passive,and constrains movement of the arm with one of said rotational degreesof freedom to within predetermined limits, while eliminating movement ofthe arm with the remaining two rotational degrees of freedom. In afurther preferred embodiment, the constraining means is entirelypassive, and eliminates movement of the arms with all three rotationaldegrees of freedom.

Measurement of the displacement of the arm with the available degrees offreedom may be detected, to the extent desired, in any convenientmanner. When rotational movement of the arm is constrained to withinpredetermined limits, detection of rotational displacement may benecessary depending upon the function which the machine is desired toperform. Linear displacement may, for example, be detected by lasertriangulation, by transducers provided within the struts, or by theprovision of a corresponding number of unpowered, or “passive”telescopic struts, universally pivotally connected to the arm and thefixed structure, and containing transducers.

One advantage of a machine according to the present invention is that ofa simplified construction, due to a reduction in the number oftelescopic struts employed. A further advantage relates to thecomparative ease and simplicity of controlling movement of the arm inreal time, due to the simple geometry of the device, i.e. movement ofone plane (defined by the three points of connection of the three strutsat one end) relative to another plane (defined by the three points ofconnection of the three struts at the other end). These advantages arenot however essential for the performance of the invention, nor are theynecessarily the only advantages of one or more of the embodimentsdescribed.

In an alternative embodiment two additional telescopic struts areprovided, each of which is connected between a mechanical earth and apoint on the movable arm remote from the mounting point of the threesupporting struts, the two additional struts controlling movement of thearm about two rotational axes, thereby converting the machine to a fiveaxis machine.

The fixed structure of the machine may be provided by a frame rigidlyconnected to a table of the machine from which the supporting struts aresuspended.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will w be described, by way of example, andwith reference to the accompanying drawings in which:

FIG. 1 shows a plan view of a first embodiment of the present invention;

FIG. 2 shows a section on the line II—II in FIG. 1;

FIG. 3 shows a detail of FIGS. 1 and 2;

FIG. 4 is a plan view of a second embodiment of the present invention;

FIG. 5 is a schematic perspective view of a detail of FIG. 4;

FIG. 6A and FIG. 6B are perspective views of a modification to theembodiment of FIGS. 4 and 5;

FIG. 7 is a plan view of a third embodiment of the present invention;

FIG. 8 is a sectional view illustrating a modification of the embodimentof FIG. 6;

FIG. 9 is a perspective view of a fourth embodiment of the presentinvention;

FIGS. 10A-D illustrate the operation of a first part of the constraintof FIG. 8;

FIGS. 11A-D illustrate the operation of a second part of the constraintof FIG. 8;

FIG. 12 is a perspective view of an alternative to the embodiments ofFIGS. 9 to 11;

FIG. 13 is a perspective view of a fifth embodiment of the presentinvention; and

FIG. 14 is a plan view on XII—XII in FIG. 11.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIGS. 1 and 2, a coordinate positioning machine, whichin the present example is a machine tool, includes an arm or movablestructure 10 in the form of a spindle, movable relative to a table 12.The spindle 10 is suspended from a rigid triangular frame 14 by means ofthree powered telescopic supporting struts 16, which extend from theapexes of the triangular frame 14 to the spindle casing 18 (in which thespindle shaft 18A is journalled). The struts also contain transducers(not shown) which measure their length; the transducers may be providedfor example by opto-electronic or magnetic encoders, LVDT's, or laserinterferometers. The supporting frame 14 is rigidly mounted to the table12 by a suitable structure which has been omitted here for clarity. Bothstructures, however, are part of the “mechanical earth” of the machine,and this is indicated throughout the specification by the usual symbol.The spindle shaft 18A carries an operating module in the form of acutting tool T, for machining workpieces (although other operatingmodules may be used, such as touch trigger and analogue probes).Preferably, the geometry of the machine is such that the axes S of eachof the supporting struts 16 intersect at the tool tip.

The connections of the supporting struts 16 to the frame 14 and thespindle casing 18 preferably permit universal pivotal motion of thestruts 16 relative to the frame 14 and casing 18. Preferably, theconnections provide substantially friction free movement, and maycomprise magnets and fluid bearings. Alternatively, flexible linkagesmay be used. Suitable connections are disclosed in our co-pendingInternational Patent Application No. PCT/GB94/02593. Translation of thespindle 10 is provided by expansion and/or contraction of thetelescoping-supporting struts 16; e.g., a simultaneous equal contractionof all of the supporting struts 16 will cause the spindle 10 to move ina direction indicated in FIG. 2 as the Z direction, with othercombinations of expansion and contraction providing movements in the Xand Y directions respectively as desired.

Because the spindle 10 is suspended by only three telescopic supportingstruts 16, the spindle may, for a given combination of strut lengths,rotate about three perpendicular axes relative to the table 12 by virtueof the universal pivotal mounting of the struts 16 relative to the frame14 and spindle casing 18. Movement of the spindle with each of thesethree degrees of rotational freedom is eliminated by the provision of ananti-rotation device having three mechanical linkages 20, 22, 24 whichprevent rotation about the X, Y and Z axes, respectively. Each of thelinkages is passive, i.e. has no motor or other actuator. One suchlinkage is illustrated in more detail in FIG. 3. Referring now to FIG.3, an individual linkage includes a substantially rigid planar member30, mounted to a mechanical earth of the machine, such as the supportingframe 14 or the table 12. The rigid member 30 has a region of relativeweakness 32 at its base, which serves as a hinge to enable tilting ofthe upper part of the member 30 about an axis Al. The upper end of themember 30 is connected to the spindle casing 18 by means of two elongaterods 34, which are flexible in bending but rigid in tension andcompression.

The operation of an individual linkage will now be described.Translational movement of the spindle casing 18 along the axis A3 ispermitted by tilting of the member 30 about axis Al, while the resultantchanging angle between the rods 34, rigid member 30 and the spindlecasing 18 is accommodated by flexing of the rods 34. Translationalmovement of the spindle casing 18 in directions parallel to either axisAl or axis A2 is permitted by flexing of the rods 34 in a manner similarto that of a pair of parallel leaf springs. Rotation of the spindlecasing 18 about an axis parallel either to the axis A1 or to the axis A2is permitted by flexing of the rods 34. The rigidity of the rods 34 totension and compression, together with the relative rigidity of themember 30 prevents rotation of the spindle casing 18 about an axisparallel to axis A3. An individual linkage 20, 22, 24 thus permitslinear movement of the spindle casing 18 in three perpendiculardirections, together with rotation thereof about two perpendicular axes,while preventing rotation about a third axis.

Referring again to FIGS. 1 and 2 it can be seen that the combined actionof all three linkages 20, 22, 24 eliminates all rotational movement ofthe spindle 10 relative to the table 12, while permitting linearmovement thereof due to telescoping of the struts 16.

In a modification of the linkage shown in FIG. 3 the planar member 30 istotally rigid, and a mechanical low-friction hinge is provided in theplace of the area of weakness 32. Additionally the elongate rods 34 arereplaced by stiff rods, universally pivotally connected to both themechanical earth and the spindle casing 18. The choice of flexural orpivoting linkages depends upon a number of factors, and particularlyupon the range of travel of the spindle casing 18 over which constraint,and in this particular embodiment, elimination of rotational movement ofthe spindle is required. Flexural linkages have the advantage of beingfriction and backlash free, but have the disadvantage that they are onlyoperable over a short range; pivoting linkages are operable over a largedistance, but suffer from friction and backlash.

A second embodiment of the present invention will now be described withreferences to FIGS. 4 and 5. A coordinate positioning machine in theform of a machine tool includes a supporting frame 114 rigidly mountedto a table (not shown). A movable arm in the form of a spindle 110 issuspended from the frame 114 by means of three powered telescopic struts(not shown). The struts are universally pivotally connected to the frame114 and spindle 110, thereby allowing, for any given combination ofstrut lengths, rotational movement of the spindle with three degrees offreedom. Movement of the spindle with two of these rotational degrees offreedom is eliminated, and movement of the spindle with a third degreeof rotational freedom is constrained to within predetermined limits byan anti-rotation device provided by a linkage 140.

The linkage 140 includes a torsionally rigid box 150, mounted by a hingeto a mechanical earth 152, thereby to enable pivoting of the box 150about axis B1. Box 150 includes top and bottom kite-shaped sub-frames154, 156 interconnected by four vertical rods 158A, B, C, D and twoangularly extending stanchions 160. Two triangular wishbone frames 162are connected at their apexes to the upper and lower ends of verticalrod 158A by means of a suitable mounting providing universal pivotalmotion thereof. The ends of the wishbone frames 162 remote from the rod158A are connected to the spindle casing 118. The linkage thus functionsin a manner similar to an elbow joint, with translation of the spindle110 in the X and Y directions being accommodated by pivoting of thetorsion box 150 about axis B1 and/or pivoting of the wishbone frames 162about axis B2. Translation of the spindle 110 in the Z direction isaccommodated by pivoting of the wishbone frames 162 as illustrated withdashed lines in FIG. 5.

The mechanical earth to which the rigid box 150 is mounted is providedat the upper end of the box by the supporting frame 114, and at itslower end by a table (not shown). When the spindle 110 occupies aposition centrally within the frame 114, the rigid torsion box 150 willextend underneath one of the spars 114A of the frame 114, while thewishbones 162 will extend substantially perpendicularly thereto. Thearcuate motions permitted by the hinged mounting of the rigid torsionbox 150 and wishbone frames 162 are illustrated in FIG. 4, and havereference numbers C1 and C2, respectively.

This anti-rotation device has the advantage of relative simplicity whencompared with the device having three individual linkages 20, 22, 24illustrated in FIGS. 1-3. However, rotation of the wishbone frames 162through a given angle, enabling movement of the spindle to occur alongthe arcuate path C2 will cause a corresponding rotation of the spindlecasing 118 about an axis parallel to the Z axis. This will beinsignificant compared with the relatively rapid rotation of the spindleshaft 118A relative to the casing 118. (The limits to within whichrotation of the spindle casing is confined are defined by thepermissible range of rotation of the wishbones 162 relative to the frame114).

If it is desired to use this construction for a coordinate measuringmachine, for example, where the permissible rotation may be significant,it may be necessary to measure the extent of rotation of the arm 110 byproviding transducers which determine the angular displacement about theaxes B1 and B2.

Referring now to FIGS. 6A and 6B, in a modification to the embodimentsof FIGS. 4 and 5, the stanchions 160 of torsion box 150 are removed. Intheir place, a pyramidal, torsionally resistant sub-frame 170 isinserted in the plan of vertical rods 158B,C. The sub-frame isuniversally pivotally mounted via ball joints 172 to the top and bottomkite-shaped sub-frames 154, 156 and therefore enables relative linearmovement between sub-frames 154, 156 and relative rotation about hingesdefined by the points of connection between sub-frames 154 and 170, and156 and 170 respectively provided by ball joints 172, but eliminates allother relative rotation and translation. Lower sub-frame 156 isuniversally pivotally mounted to a two-axis linear stage 180, movablewith two linear degrees of freedom relative to the mechanical earth, onwhich upper sub-frame 154 is universally pivotally mounted. Referringspecifically to FIG. 6B, movement of the two-axis stage will result in achange in the angle of orientation of the spindle casing 118. Thismodification has a number of applications. For example, the two-axisstage 180 may be used only during set-up of e.g. a machine tool, toensure that the axis of the spindle casing 118 lies orthogonal to theplane of the table relative to which spindle casing 118 in movable.Alternatively, the stage 180 may be employed during normal operation ofthe machine, e.g. during machining, to provide two-axis rotationalorientation of the tool. Alternatively, the two-axis stage 180 may beemployed to ensure that the spindle casing 118 is correctly orientedrelative to any given workpiece to be machined or inspected. Forexample, in the case of a machining operation, a workpiece may be probedin advance in order to determine the plane of, e.g. its upper surface,and subsequently the stage 180 may be actuated to adjust the axis of thespindle casing 118, such that the plane of the workpiece surface andspindle casing axis are orthogonal.

A third embodiment of the present invention will now be described withreference to FIGS. 7 and 8. As with previous embodiments the machineincludes a spindle 210 supported relative to a table 212 and a frame 214by three powered telescopic universally pivotally connected struts 216.Movement of the spindle 210 with the resultant three rotational degreesof freedom is constrained by a combination of a single passive linkage220, of the type illustrated in FIG. 3, and a pair of powered auxiliarytelescopic constraining struts 280, 282. The passive linkage 220eliminates rotational movement of the spindle casing 218 about the Zaxis. Rotational of the spindle 210 about X and Y axes is controlled bythe two auxiliary struts 280, 282, which are connected at one end to amechanical earth, and at the other end to an elongate pillar 286, whoselower end is rigidly connected to the spindle casing 218. Telescopingexpansion and contraction of the auxiliary struts 280, 282 thus causesrotation of the spindle 210 about the X and Y axes. In the embodiment ofFIG. 7, the auxiliary struts 280, 282 are pivotally connected to amechanical earth distinct from the frame 214, and extend insubstantially perpendicular directions. In the alternative embodiment ofFIG. 8, however, the auxiliary struts 280, 282 are pivotally mounted toa mechanical earth provided by two of the apexes of the supporting frame214. The provision of the auxiliary struts 280, 282 has the effect ofconverting a machine having three linear degrees of freedom to a machinewith five degrees of freedom, three of which are linear and two of whichare rotational.

A fourth embodiment will now be described with reference to FIGS. 9 to11. The machine includes an arm 310 movable relative to a table (notshown), a supporting frame 314, and three powered telescopic struts 316,universally pivotally connected at their ends to the arm 310 and thesupporting structure 314, thereby supporting the arm 310 in a mannerwhich, for a given combination of strut lengths, permits rotation of thearm 310 with three degrees of freedom.

Constraint of each of these three rotational degrees freedom is providedby a passive anti-rotation device 340, which eliminates each of thesedegrees of rotational freedom while permitting three dimensionaltranslation of the arm under the action of the powered struts 316. Theanti-rotation device 340 acts between a mechanical earth and the movablearm 310, and includes a linkage provided by an extensible torsion box350, which prevents rotation of the arm 310 about two mutuallyorthogonal axes A, B together with an auxiliary rotational constraininglinkage 360, which prevents rotation of the arm 310 about a third axis,C, extending orthogonal to both axes A and B.

Referring now to FIG. 10A, the extensible torsion box 350 includes twodoor members 352 each of which is mounted to the mechanically earthedstructure at a common hinge 354. The movable arm 310 is connected to thedoor members 352 at two points spaced apart in the Z-direction by meansof two pairs of V-shaped connecting rods 356A, 356B. Each of the pairsof connecting rods 356A, 356B are universally pivotally connected (e.g.by ball joints) to the movable arm at their apex, with the pairs ofconnecting rods 356A, 356B lying vertically in register with each other.The ends of the pairs of rods 356A, 356B remote from the point ofconnection with the arm 310 are universally pivotally connected to thecorners of free swinging edges on door members 352.

Referring now additionally to FIGS. 10B-D, the extensible torsion box350 permits linear movement of the movable arm 310 in one or more of thedirections X, Y, and Z, while simultaneously preventing rotation of thearm about rotational axes A and B. Movement in any one of the X, Y and Zdirections is enabled by a combination of pivoting movements of theelements 352, and 356A, 356B relative to each other, and the mechanicalearth. For example, referring now to FIG. 10B, translational movement ofthe movable arm 310 in the X direction is enabled by inward or outwardpivoting movement of the door members 352 about hinge 354, pivoting ofthe connecting rods 356A, 356B relative to their points of connectionwith the movable arm 310 and the door members 352, and additionally aswinging movement of the door members 352, and additionally a swingingmovement of the door members 352 about hinge 354. Referring now to FIGS.10C and D, translational movement in either the Z or the Y direction isprovided by a combination of either simultaneous inward or outwardpivoting of the door members 352 relative to each other and about hinge354, and pivoting movement of the connecting rods 356A, 356B about theirpoints of connection with the movable arm 310 and the door members 352.

The universal pivotal connection of the rods 356A, 356B to the movablearm 310 enables arcuate translational motion of the movable arm 310about the axis C (on which the points of connection of the rods 356A,356B with the arm 310 lie) and, as a result, a corresponding limiteddegree of rotation of the arm 310. This minor rotational freedom of themovable arm 310 is constrained by the linkage 360 illustrated in FIGS.11A-D. Referring now to FIG. 11A, the linkage 360 includes a first pairof fixed rods 362 which are rigidly connected at one end to themechanical earth, and are universally pivotally connected at the otherby ball joints (not shown) to a further rigid door member 364. Thefurther door member 364 is thus hinged for movement relative to theearth about an axis D. A pair of movable rods 366 are universallypivotally connected to the lower end of further door member 364 and themovable arm 310. The rigidity of the rods 362, 366 and further doormember 364 prevents rotation of the arm about the axis C, while theuniversal pivotal connection between the arm 310, movable rods 366,further door member 364, and fixed rods 362 allows translational motionin the X, Y and Z directions as illustrated in FIGS. 11A-D.

All rigid members of the constraining device 340 which are universallypivotally mounted, may be replaced by flexural elements, as appropriate.

Referring now to FIG. 12, in an alternative to the embodiment describedin FIGS. 9-11, constraint of all three rotational degrees of freedom ofthe spindle casing 318 is provided by an anti-rotation device whichincludes a linkage having upper and lower kite-shaped sub-frames 374,376 and an intermediate, torsionally resistant pyramidal sub-frame 380,upon which upper and lower sub-frames 374, 376 are pivotally mounted atball joints 382. Upper and lower sub-frames 374, 376 may be pivotallymounted upon ball joints 392, provided on the mechanical earth, or fixedstructure, the tension springs 378 enabling easy “snap-on” mounting.Also mounted to the mechanical earth or fixed structure are a pair ofelongate substantially rigid struts 384, 386; the ends of the sub-frames374, 376 and the struts 384, 386, distal to the mechanical earth areuniversally pivotally mounted, via ball joints 390, to an intermediatelinkage member 388. The intermediate linkage member 388 is connected tothe spindle casing 318 via two rigid door members 394, 396, each ofwhich is mounted, via hinges, 397A, B and 398A, B to the intermediatelinkage member 388 and spindle casing 318, respectively. In use,translational movement of the spindle casing 318 is permitted in the XYplane by pivoting of the various elements of the constraint about hingesillustrated with the reference numerals E, F, G, H, I, J, K whiletranslation of the spindle 318 in the Z direction is provided bypivoting of sub-frame 374, 376 and linkages 384, 386 about hingesillustrated by the reference numerals P, Q, R, S.

A fifth embodiment of the present invention will now be described withreference to FIGS. 13-14. The machine of FIG. 13 has a movable arm 410,table 412, fixed supporting structure 414, and three powered telescopicstruts 416, universally pivotally connected at their ends to thesupporting structure 414 and the arm 410.

Rotation of the arm 410 about the Z axis is eliminated, and rotationabout the X and Y axis is constrained to within predetermined limits bymeans of motion control linkages 470, 480. Motion control linkage 470 isconnected at one end to a mechanical earth, and at the other to the arm410, by means of ball joints 472, 474 respectively. Motion controllinkage 480 has the form of a fork which extends at right angles tomotion control linkage 470, and is universally pivotally connected atone end to a mechanical earth by a ball joint 482, and at the other todiametrically opposing sides of the arm 410 by means of bearings 484.(In an alternative embodiment, ball joints may be used.)

For a given static setting of telescopic struts 416, motion controlstruts 470, 480 prevent rotation of the spindle 410. Upon actuation ofone or more of the telescopic struts 416, the free end of the arm 410may be positioned in a desired location by a combination of translationof the arm 410, together with arcuate movement thereof as a result ofthe action of motion control struts 470, 480. For example, simultaneousexpansion or contraction of the telescopic struts 416 will result inlinear movement of the arm 410 in the Z direction, in combination withan arcuate movement of the upper and lower ends of the arm 410,resulting from the consequential pivoting of motion control struts 470,480. Similarly, movement of the telescopic struts 416 to translate thearm 410 in the X direction will additionally result in rotation of thearm 410 about an axis parallel to the Y axis, at a point defined by theposition of the ball joint 474 and bearings 484; movement of thetelescopic struts 416 to execute a translation of the arm 410 in the Ydirection will result additionally in an arcuate pivoting of the arm 410about an axis parallel to the X axis and a point defined by ball joint474 and bearings 484.

Because translational movement of the arm 410 will inevitably result ina change in the orientation of its free end, a two axis robot-wrist willpreferably be mounted upon the arm 410, and more preferably a three-axiswrist; this enabling machining of a part at a plurality of angles andorientations.

Machines actuated by three telescopic struts are easier to control thanprior art machines with six struts. This is because the mathematicsinvolved in controlling the motion of the movable arm is simpler. Inparticular, the algorithms required to determine the position of thearms are now based only upon three linear measurements, rather than six.

What is claimed is:
 1. A coordinate positioning machine, comprising:first and second relatively movable structures interconnected by threeextensible and retractable struts, each of the struts being connected tothe first and second structures by first and second pivotal mountingsrespectively, the mountings enabling pivoting motion of the strutsrelative to the structures with at least two degrees of rotationalfreedom; each of the struts having a length defined by a distancebetween the respective first and second pivotal mountings, relativetranslational motion of the first and second structures being actuatedby a change in length of one or more of the struts; and a passiveanti-rotation device that passively eliminates at least two degrees ofrotational freedom of the first structure relative to the secondstructure; wherein said passive anti-rotational device includes at leastone mechanical linkage connected at one end to a first one of thestructures and at the other end to the other one of the structures, themechanical linkage having a plurality of links each of which is rigid intension and compression.
 2. A coordinate positioning machine accordingto claim 1, wherein one of the structures is a fixed structure, theother one of the structures is a moveable structure.
 3. A coordinatepositioning machine according to claim 2, wherein the passiveanti-rotation device comprises two mechanical linkages and eliminatesall three degrees of rotational freedom of the moveable structurerelative to the fixed structure.
 4. A coordinate position machineaccording to claim 3, wherein the passive anti-rotation device comprisesthree mechanical linkages, and eliminates three degrees of rotationalfreedom of the moveable structure relative to the fixed structure.
 5. Acoordinate positioning machine according to claim 2, further comprisingthree transducers for measuring displacement of the moveable structurerelative to the fixed structure, each of the transducers acting directlybetween the fixed structure and the moveable structure.
 6. A coordinatepositioning machine according to claim 5, wherein the transducers areindependent of the struts.
 7. A coordinate positioning machine accordingto claim 1, wherein said at least one mechanical linkage comprises atorsionally rigid box connected to one of the structures by a hingewhich allows one degree of rotational freedom of the box, and a pair oflinks which are each universally pivotally mounted at one end to the boxand at the other end to the other of the structures.
 8. A coordinatepositioning machine according to claim 7, wherein each of the links ofsaid pair comprises a triangular frame.
 9. A coordinate positioningmachine according to claim 7, wherein one of the structures is a fixedstructure, the other one of the structures is a moveable structure, andthe box is pivotally connected to the fixed structure by means of saidhinge.
 10. A coordinate positioning machine according to claim 7,wherein one of the structures is a fixed structure, the other one of thestructures is a moveable structure.
 11. A coordinate positioning machineaccording to claim 10, wherein the at least one mechanical linkagecomprises a torsionally rigid box including two door members, both ofwhich are independently pivotally connected to the fixed structure by ahinge, and two pairs of connecting rods, each of said rods beinguniversally pivotally connected at one of its ends to a door member andat the other of its ends to the moveable structure.
 12. A coordinatepositioning machine according to claim 1, wherein each mechanicallinkage comprises a torsionally rigid member mounted by a hinge to oneof the structures, and a pair of links, each of the links of a said pairof links being connected at one of its ends to the torsionally rigidmember and at the other of its ends to the other one of the structures.13. A coordinate positioning machine according to claim 12, wherein oneof the structures is a fixed structure and the other one of thestructures is a moveable structure to which an operating module may beconnected, and the torsionally rigid member is mounted by said hinge tothe fixed structure to allow only one degree of rotational freedom ofsaid member relative to the fixed structure.
 14. A coordinatepositioning machine according to claim 13, wherein said passiveanti-rotational device comprises two torsionally rigid members, each ofwhich is pivotally mounted by a hinge to the fixed structure and each ofwhich is connected by a pair of links to the moveable structure.
 15. Acoordinate positioning machine according to claim 14, wherein thetorsionally rigid members are pivotally mounted to the fixed structureat a common hinge.
 16. A coordinate positioning machine according toclaim 14, wherein the torsionally rigid members are door members each ofwhich is pivotably mounted to the fixed structure at a common hinge. 17.A coordinate positioning machine according to claim 14, wherein saidpassive anti-rotational device comprises three torsionally rigidmembers, each of which is pivotally mounted with one degree ofrotational freedom to the fixed structure, and each of which has a pairof links connected between the moveable structure and the torsionallyrigid members.
 18. A coordinate positioning machine, comprising: a fixedstructure; a moveable structure which is moveable relative to the fixedstructure, and upon which an operating module may be mounted; the fixedand moveable structures being interconnected by three extensible andretractable struts, each of the struts being connected to the fixed andmoveable structures by first and second pivotal mountings respectively,the mountings enabling pivoting motion of the struts relative to thestructures with at least two degrees of rotational freedom; each of thestruts having a length defined by a distance between the respectivefirst and second pivotal mountings, relative translational motionbetween the fixed and moveable structures being actuated by a change inlength of one or more of the struts; and a passive anti-rotation devicethat passively eliminates at least two degrees of rotational freedom ofthe moveable structure relative to the fixed structure, wherein saidpassive anti-rotation device includes at least one mechanical linkageconnected at one end to the fixed structure and at the other end to themoveable structure, the mechanical linkage having a plurality of linkseach of which is rigid in tension and compression.
 19. A coordinatepositioning machine according to claim 18, wherein the mechanicallinkage comprises a first link pivotally connected to the fixedstructure and a second link pivotally connected at one of its ends tothe moveable structure and at the other one of its ends to the firstlink.
 20. A coordinate positioning machine according to claim 19,wherein the first link is connected to the fixed structure by means of apivot which allows only one degree of rotational freedom.
 21. Acoordinate positioning machine according to claim 20, wherein the secondlink comprises a pair of struts each of which is universally pivotallyconnected to both the first link and to the moveable structure with twodegrees of rotational freedom.
 22. A coordinate positioning machineaccording to claim 19, wherein the first link comprises a rigid doormember.
 23. A coordinate positioning machine according to claim 22,wherein the second link comprises a pair of struts each of which isuniversally pivotally connected to both the first link and to themoveable structure with two degrees of rotational freedom.
 24. Acoordinate positioning machine according to claim 18, wherein said atleast one mechanical linkage comprises a torsionally rigid box connectedto the fixed structure by a hinge which allows one degree of rotationalfreedom of the box, and a pair of links which are each universallypivotally mounted at one end to the box and at the other end to themoveable structure.
 25. A coordinate positioning machine according toclaim 24, wherein each of the links of said pair comprises a triangularframe.
 26. A coordinate positioning machine according to claim 18,wherein the at least one mechanical linkage comprises a torsionallyrigid box including two door members, both of which are independentlypivotally connected to the fixed structure by a hinge, and two pairs ofconnecting rods, each of said rods being universally pivotally connectedat one of its ends to a door member and at the other of its ends to themoveable structure.
 27. A coordinate positioning machine, comprising:first and second relatively moveable structures interconnected by threeextensible and retractable struts, each of the struts being connected tothe first and second structures by first and second pivotal mountingsrespectively, the mountings enabling pivoting motion of the strutsrelative to the structures with at least two degrees of rotationalfreedom; each of the struts having a length defined by a distancebetween the respective first and second pivotal mountings, relativetranslational motion of the first and second structures being actuatedby a change in length of one or more of the struts; and an anti-rotationdevice that eliminates at least two degrees of rotational freedom of thefirst structure relative to the second structure; wherein saidanti-rotation device includes at least one mechanical linkage connectedat one end to a first one of the structures and at the other end to theother one of the structures, the mechanical linkage having a pluralityof links each of which is rigid in tension and compression.
 28. Acoordinate positioning machine according to claim 27, wherein eachmechanical linkage comprises a torsionally rigid member mounted by ahinge to one of the structures, and a pair of links, each of the linksof a said pair of links being connected at one of its ends to thetorsionally rigid member and at the other of its ends to the other oneof the structures.
 29. A coordinate positioning machine according toclaim 28, wherein one of the structures is a fixed structure and theother one of the structures is a moveable structure to which anoperating module may be connected, and the torsionally rigid member ismounted by said hinge to the fixed structure to allow only one degree ofrotational freedom of said member relative to the fixed structure.
 30. Acoordinate positioning machine according to claim 29, wherein saidpassive anti-rotational device comprises two torsionally rigid members,each of which is pivotally mounted by a hinge to the fixed structure andeach of which is connected by a pair of links to the moveable structure.31. A coordinate positioning machine according to claim 30, wherein thetorsionally rigid members are pivotally mounted to the fixed structureat a common hinge.
 32. A coordinating positioning machine according toclaim 30, wherein the torsionally rigid members are door members each ofwhich is pivotably mounted to the fixed structure at a common hinge. 33.A coordinating positioning machine according to claim 30, wherein saidpassive anti-rotational device comprises three torsionally rigidmembers, each of which is pivotally mounted with one degree ofrotational freedom to the fixed structure, and each of which has a pairof links connected between the moveable structure and the torsionallyrigid members.